What is wrong with the zipper pull?
If the tab is gone, but the slider works properly you can easily replace the tab.
If the slider doesn't work properly, maybe the zipper pulls apart. Then the slider needs replaced. This is not too hard. You will probably need to pull some stitching at the top of the zipper and then pull the current slider out. You can then slide the new one on and restitch the top. The biggest issue is getting the right size and type of slider.
You can find replacement zipper parts here:
http://www.hardwareelf.com:8080/elf/zippers.jsp
Mike
March 2010